Gallium nitride (GaN)-based high electron mobility transistors (HEMTs) with a thin ternary or a quaternary barrier, particularly AlGaN/GaN HEMTs formed with aluminum gallium nitride (AlGaN), have found promising applications as high frequency, high electric field, high power devices. Such devices typically include an AlGaN barrier layer formed on a GaN channel/buffer layer, with a gate formed on an upper surface of the AlGaN layer. A two-dimensional (2D) electron gas is formed at the AlGaN/GaN interface due to tight quantum confinement of the density of states in the third dimension. This quantization of states at the interface effectively works as a two dimensional model of confined electrons. Electrons (or holes) can move with high mobility in the plane of the AlGaN/GaN interface, however, vertical motion is confined by the band structure of the heterojunction and can be ignored in practical applications.
One advantage of AlGaN/GaN HEMTs is the proximity of this 2D electron gas to the surface of the HEMT, which leads to a low contact resistance Ohmic contacts to the 2D electron channel. However, despite the proximity of this 2D electron gas to the surface, device reliability problems can arise when electrons can become trapped in surface defects, causing carrier scattering manifested by reduced output current, i.e., current collapse.
FIG. 2A illustrates the concept of quiescent bias well-known to those skilled in the art. The device is stressed at pre-determined values for the drain and gate voltage (VGS,Q and VDS,Q), after which dynamic on resistance (RON,DYN) is measured before the quiescent bias conditions are applied again. This method provides device performance information under more realistic operating conditions. For example, FIG. 2B shows the reduction in output current, and corresponding increase in RON,DYN for VDS,Q up to 50 V.
Carrier scattering in GaN-based HEMTs has been traditionally mitigated by means of a passivation dielectric such as SiN, AlN, SiON, SiO2, MgO, Sc2O3, HfO2 deposited on the upper surface of the HEMT. Of these methods, SiN passivation is most common. See A. P. Edwards, J. A. Mittereder, S. C. Binari, D. S. Katzer, D. F. Storm, and J. A. Roussos, “Improved reliability of AlGaN—GaN HEMTs using an NH3 plasma treatment prior to SiN passivation,” IEEE Electr. Dev. Lett., Vol. 26, No. 4, pp. 225 (2005); S. C. Binari, K. Ikossi, J. A. Roussos, W. Kruppa, D. Park, H. B. Dietrich, D. D. Koleske, A. E. Wickenden, and R. L. Henry, “Trapping Effects and Microwave Power Performance in AlGaN/GaN HEMTs,” IEEE Trans. Electr. Dev., Vol. 48, No. 3, pp. 465-471 (2001); H. Kim, R. M. Thompson, V. Tilak, T. R. Prunty, J. R. Shealy, and L. F. Eastman, “Effects of SiN passivation and high-electric field on AlGaN—GaN HFET degradation,” IEEE Electr. Dev. Lett., Vol. 24, No. 7, pp. 421-423 (2003); and X. Wang, S. Huang, Y. Zheng, K. Wei, X. Chen, G. Liu, T. Yuan, W. Luo, L. Pang, H. Jiang, J. Li, C. Zhao, H. Zhang, and X. Liu, “Robust SiNx/AlGaN Interface in GaN HEMTs Passivated by Thick LPCVD-grown SiNx Layer,” IEEE Electr. Dev. Lett., Vol. 36, No. 7, pp. 666-668 (2015).
Other methods for obtaining improved AlGaN/GaN HEMT surface passivation that have been used in the prior art include low pressure CVD (LPCVD) of SiN, see Wang et al., supra; molecular beam epitaxy (MBE), see B. P. Downey, et al., “Effect of SiNx gate insulator thickness on electrical properties of SiNx/In0.17Al0.83N/AlN/GaN MIS-HEMTs,” Solid State Electron., Vol. 106, pp. 12-17 (2015); and metal organic CVD (MOCVD), see M. J Tadjer, et al., “Electrical and Optical Characterization of AlGaN/GaN HEMTs with In Situ and Ex Situ Deposited SiNx Layers,” J. Electr. Mater., Vol. 39, No. 11, pp. 2452-2458 (2010).
Mixed-frequency plasma-enhanced CVD (PECVD) SiN deposition is another common method used for passivation of GaN-based HEMTs. A typical such passivated HEMT is shown in FIG. 1, and includes an AlGaN/GaN HEMT 101 having a gate 102 formed thereon, with a 100 nm-thick high-frequency/low-frequency SiN pas sivation layer deposited on the upper surface of the HEMT.
However, low-frequency plasma in mixed-frequency PECVD SiN deposition can introduce undesirable damage to the surface of the III-Nitride heterostructure and consequently degrade device performance. This plasma damage originates from the ion energy distribution function in a radio frequency (RF) generated plasma, which is frequency dependent. Specifically for PECVD, at a low-frequency (LF, 100-360 kHz) both electrons and ions are energized towards the GaN surface by the RF plasma, whereas the ion energies are much lower during high frequency (HF, 13.56 MHz) rf power. As a result, additional surface traps on the AlGaN surface can be created by ions energized from the LF plasma.
There have been attempts to address this problem of N ions bombarding the AlGaN surface during low frequency plasma deposition. See W. S. Tan and coworkers in 2004. See W. S. Tan, P. A. Houston, G. Hill, R. J. Airey, and P. J. Parbrook, “Influence of Dual-Frequency Plasma-Enhanced Chemical-Vapor Deposition Si3N4 Passivation on the Electrical Characteristics of AlGaN/GaN Heterostructure Field-Effect Transistors,” J. Electr. Mater., Vol. 33, No. 5, pp. 400 (2004).
No methods to date have been able to achieve SiN passivation without damage to the HEMT surface.